Poseidonis is the fictional last remnant of the lost continent of Atlantis. Algernon Blackwood mentioned it in his novella “Sand,” published in the 1912 collection Pan’s Garden, and Clark Ashton Smith detailed it in a series of short stories, basing it on Theosophical writings about Atlantis such as Helena Blavatsky’s The Secret Doctrine.
Smith’s Poseidonis cycle includes “The Last Incantation,” “The Death of Malygris,” “The Double Shadow,” “A Voyage to Sfanomoë” and “A Vintage from Atlantis,” along with prose poems, and was gathered by Lin Carter into a Ballantine volume in 1973. It sits beside Hyperborea, Zothique and Averoigne among Smith’s settings. His idea of “the last isle of foundering Atlantis” is echoed by Tolkien’s Númenor.